The presently disclosed embodiments are directed to an imaging member used in electrophotography. More particularly, the embodiments pertain to a flexible electrophotographic imaging member with an improved anticurl back coating layer comprising a low surface energy polymeric material and a process for making and using the member.
In electrophotographic or electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Flexible electrostatographic imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. The flexible electrostatographic imaging members may be seamless or seamed belts; and seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. Typical electrophotographic imaging member belts include a charge transport layer (CTL) and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating coated onto the opposite side of the substrate layer. A typical electrographic imaging member belt does, however, have a more simple material structure; it includes a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Although the scope of the present invention covers the preparation of all types of flexible electrostatographic imaging members, however for reason of simplicity, the discussion hereinafter will focus only on flexible electrophotographic imaging members.
Electrophotographic flexible imaging members may include a photoconductive layer including a single layer or composite layers. Since typical flexible electrophotographic imaging members exhibit undesirable upward imaging member curling, an anti-curl back coating, applied to the backside, is required to balance the curl. Thus, the application of anti-curl back coating is necessary to provide the appropriate imaging member with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer (CTL). Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous CTL and the supporting conductive layer. Alternatively, the CTL may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge-generating layer (CGL) is sandwiched between the outermost exposed CTL and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of Gen layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In flexible web like photoreceptor the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, a CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anticurl back coating layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, charge generating layer, CTL and other layers.
Typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer (CGL), a charge transport layer (CTL). The CTL is usually the last layer to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 115° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the CTL coating through drying/cooling process, upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the CTL in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the CTL has a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature. The exhibition of imaging member curling after completion of CTL coating is due to the consequence of the heating/cooling processing, according to the mechanism: (1) as the web stock carrying the wet applied CTL is dried at elevated temperature, dimensional contraction does occur when the wet CTL coating is losing its solvent during 115° C. elevated temperature drying, because the CTL at 115° C. still remains as a viscous liquid after losing its solvent. Since its glass transition temperature (Tg) is about 85° C. the CTL will flow to automatically re-adjust itself to compensate the losing of solvent and maintain its dimension; (2) as the CTL in a viscous liquid state is cooling down further and reaching its Tg at 85° C. the CTL instantaneously solidifies and adheres to the CGL because it has transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) cooling down the solidified CTL of the imaging member web from 85° C. down to 25° C. room ambient will then cause the CTL to contract more than the substrate support since it has an approximately 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This dimensional contraction mis-match results in tension strain built-up in the CTL, at this instant, is pulling the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock will spontaneously curl upwardly into a 1.5-inch tube. To offset the curling, an anti-curl back coating (ACBC) is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
Curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. An anticurl back coating having a counter curling effect equal to and in the opposite direction to the applied layers is applied to the reverse side of the active imaging member to eliminate the overall curl of the coated device by offsetting the curl effect which is arisen from the mismatch of the thermal contraction coefficient between the substrate and the CTL, resulting in greater CTL dimensional shrinkage than that of the substrate. Although the anticurl back coating counters and balances the curl so as to promote the imaging member web to lay flat, nonetheless, common anticurl back coating formulations are not always providing satisfying dynamic imaging member belt performance result under a normal machine functioning condition; for example, exhibition of anticurl back coating wear and its propensity to cause electrostatic charging-up are the frequently seen problems to prematurely cut short the service life of a belt and requires its frequent costly replacement in the field.
Anticurl back coating wear under the normal imaging member belt machine operational conditions reduces the anticurl back coating thickness, causing the lost of its ability to fully counteract the curl as reflected in exhibition of imaging member belt curl in the field. Curling is undesirable during imaging belt function because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, causing non-uniform charging. In addition, developer applicators and the like, during the electrophotogradhic imaging process, may all adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. =Since the anticurl back coating is an outermost exposed backing layer and has high surface contact friction when it slides over the machine subsystems of belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function, these mechanical sliding interactions against the belt support module components not only exacerbate anticurl back coating wear, it does also cause the relatively rapid wearing away of the anti-curl produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance.
Moreover, high contact friction of the anticurl back coating against machine subsystems is further seen to cause the development of electrostatic charge built-up problem. In other machines the electrostatic charge builds up due to contact friction between the anti-curl layer and the backer bars increases the friction and thus requires higher torque to pull the belts. In full color machines with 10 pitches this can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than one which are to be coordinated electronically precisely to keep any possibility of sagging. Static charge built-up in anticurl back coating increases belt drive torque, in some instances, has also been found to result in absolute belt stalling. In other cases, the electrostatic charge build up can be so high as to cause sparking.
Another problem encountered in the conventional belt photoreceptors using a bisphenol A polycarbonate anticurl back coating that are extensively cycled in precision electrostatographic imaging machines utilizing belt supporting backer bars, is an audible squeaky sound generated due to high contact friction interaction between the anticurl back coating and the backer bars. Further, cumulative deposition of anticurl back coating wear debris onto the backer bars may give rise to undesirable defect print marks formed on copies because each debris deposit become a surface protrusion point on the backer bar and locally forces the imaging member belt upwardly to interferes with the toner image development process. On other occasions, the anticurl back coating wear debris accumulation on the backer bars does gradually increase the dynamic contact friction between these two interacting surfaces of anticurl back coating and backer bar, interfering with the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases.
Another known wear resistance anticurl back coating layer used in the printing apparatuses includes organic particles reinforcement such as polytetrafluoroethylene (PTFE) dispersed in polymer binder. PTFE is commonly incorporated to reduce the friction between the anticurl back coating of the belt and the backer bars. The benefit of this formulation, however, is out-weighted by a major drawback because of a problem with PTFE particle dispersion stability of the coating solution. PTFE, being two times heavier than the coating solution, forms an unstable dispersion in a polymer coating solution, commonly a bisphenol A polycarbonate polymer solution, and tends to settle with particles flocculate themselves into big agglomerates in the mix tanks if not continuously stirred. The dispersion problem can result in an anticurl back coating with insufficient and variable or inhomogeneous PTFE dispersion along the length of the coated web, and thus, inadequate reduction of friction over the backer bars in the copiers or printers. This causes significant complications in the larger copiers or printers, which often include so many backer bars that the high friction increases the torque needed to drive the belt. Consequently, two driving rollers are included and synchronized to prevent any misregistration. The additional components result in high costs for producing and using these larger printing apparatuses. Thus, if the friction could be reduced, the apparatus design in these larger printing apparatuses could be simplified with less components, resulting in significant cost savings. The present disclosures discussed above also contemplate dispersion of other particles, such as amorphous silica or nano particles PTFE in the solution of polymeric binder. However, these generally have a problem of creating a good particle dispersion quality consisting of only homogeneously dispersion primary particles in the resulting anticurl back coating; Moreover, the problems of instability of solutions and thus the shelf life are serious issues; and consequently, the coating solution needs to be constantly stirred. Although anticurl back coating formulated prepared only with the pure polymeric binder solution without incorporation of particle dispersion does not have the stability problem, but it does have all the inherent shortcomings, as discussed above.
In U.S. Pat. No. 5,069,993, an exposed layer in an electrophotographic imaging member is provided with increase resistance to stress cracking and reduced coefficient of surface friction, without adverse effects on optical clarity and electrical performance. The layer contains a polymethylsiloxane copolymer and an inactive film forming resin binder. Various specific film forming resins for the anti-curl layer and adhesion promoters are disclosed.
U.S. Pat. No. 5,021,309 shows an electrophotographic imaging device, with material for an exposed anti-curl layer has organic fillers dispersed therein. The fillers provide coefficient of surface contact friction reduction, increased wear resistance, and improved adhesion of the anti-curl layer, without adversely affecting the optical and mechanical properties of the imaging member.
U.S. Pat. No. 5,919,590 shows An electrostatographic imaging member comprising a supporting substrate having an electrically conductive layer, at least one imaging layer, an anti-curl layer, an optional ground strip layer and an optional overcoat layer, the anti-curl layer including a film forming polycarbonate binder, an optional adhesion promoter, and optional dispersed particles selected from the group consisting of inorganic particles, organic particles, and mixtures thereof.
In U.S. Pat. No. 4,654,284 an electrophotographic imaging member is disclosed comprising a flexible support substrate layer having an anti-curl layer, the anti-curl layer comprising a film forming binder, crystalline particles dispersed in the film forming binder and a reaction product of a bifunctional chemical coupling agent with both the binder and the crystalline particles. The use of VITEL PE 100 in the anti-curl layer is described.
In U.S. Pat. No. 6,528,226 a process for preparing an imaging member is disclosed that includes applying an organic layer to an imaging member substrate, treating the organic layer and/or a backside of the substrate with a corona discharge effluent, and applying an overcoat layer to the organic layer and/or an anticurl back coating to the backside of the substrate.
Thus, electrophotographic imaging members comprising a supporting substrate; having a conductive surface on one side, coated over with at least one photoconductive layer and coated on the other side of the supporting substrate with a conventional prior art anticurl back coating that does exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers. While the above mentioned electrophotographic imaging members may be suitable or limited for their intended purposes, further improvement on these imaging members are desirable and urgently needed. For example, there continues to be the need for improvements in such systems, particularly for an imaging member belt that includes an improved anticurl back coating which sufficiently counters curling to render flatness, reduces friction, has superb wear resistance, provides lubricity to ease belt drive, nil or no wear debris, and eliminates electrostatic charge build-up problem, even in larger printing apparatuses.